Hamilton Exteriors is headquartered in Castro Valley at 21634 Redwood Rd. We are your neighbors, not a drive-in crew. We have built ADUs throughout Castro Valley and understand the specific lot conditions, setback requirements, and county processes that apply here.
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ADU Builder in Castro Valley
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Since 2018
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For local cost factors and planning, read our ADU construction cost guide for Castro Valley.
For detailed ADU pricing factors, use our 2026 Bay Area ADU cost guide.
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Start Your ADU Consultation Online
Share your property and project goals. Our team will discuss feasibility, design, permits, and the next planning step.
ADU certifications, brands, and contractor credentials
Customer-Service Obsessed
Responsive from consultation through project closeout
Architect and general contractor
One accountable team
Licensed, bonded, and insured
Verified business credentials
ADU Services 
Start With the ADU Service Your Property Needs
Detached ADUs
A new backyard structure planned around the lot, setbacks, access, utilities, fire requirements, privacy, permits, and construction logistics.
ADU Design
Architect-led feasibility and plans shaped by the property, existing home, program, code constraints, utilities, and budget.
ADU Permits
Plan-check coordination for architecture, engineering, energy documents, utility requirements, agency comments, revisions, and inspections.
Garage Conversion ADUs
Conversion planning for the existing slab, framing, ceiling height, fire separation, insulation, utilities, windows, access, and permits.
Castro Valley is unincorporated Alameda County. ADU permits go through the Alameda County Planning Department, which follows California's statewide ADU laws (AB 68, SB 9). The county cannot require owner-occupancy for ADUs and must approve compliant applications within 60 days. Hamilton Exteriors manages the full permit process with the county.
Castro Valley's mix of flat valley lots and hillside properties creates varied ADU opportunities. Flat lots along Castro Valley Boulevard and in Proctor are ideal for detached backyard ADUs. Hillside lots in Five Canyons and Palomares Hills may require grading, retaining walls, and creative design to maximize the allowable 1,200 square feet.
Most Castro Valley lots have two-car garages that convert well into studio or one-bedroom ADUs. Garage conversions are typically the most affordable ADU option at $150,000 to $200,000. Detached new-construction ADUs range from $200,000 to $400,000 depending on size and site conditions. Junior ADUs (within the main house) start around $150,000.
Neighborhoods We Serve
ADUs Across Castro Valley Neighborhoods
Local planning starts with the site, neighborhood context, access, and applicable city review.
Crow Canyon
Crow Canyon lots range from 6,000 to over 10,000 square feet, providing ample space for detached ADUs. Hillside setbacks and grading requirements vary. We design ADUs that work with the natural slope.
Five Canyons
Five Canyons properties on hillside lots may have HOA design guidelines. We work within community standards while maximizing the allowable ADU size. Hillside ADUs here can take advantage of valley views.
Proctor
Proctor's flat lots and oversized two-car garages make this neighborhood ideal for garage-to-ADU conversions. The grid street layout provides good access for construction vehicles during the build.
Palomares Hills
Palomares Hills lots are among the largest in Castro Valley. Detached ADUs up to 1,200 sq ft fit comfortably with proper setback planning. The rural character of the area pairs well with cottage-style ADU designs.
Jensen Ranch
Jensen Ranch backyards are typically 2,500 to 4,000 square feet, providing adequate space for a detached ADU with setback compliance. We maximize unit size while preserving usable outdoor space.
Castro Valley Boulevard
Properties near Castro Valley Boulevard benefit from proximity to shops, restaurants, and BART shuttle routes. ADUs here are attractive rental units due to the walkable location and transit access.
Greenridge
Greenridge lots offer space for detached ADUs. Mature tree preservation may influence unit placement. We work with arborist reports when needed to protect significant trees during construction.
Fairview
Fairview area properties include larger lots with gentle slopes. ADUs here often serve as in-law units or rental income properties. We design units that complement the main home's architecture.
Project Management How Hamilton Exteriors
How Hamilton Exteriors
Manages Your ADU Project
Property Feasibility
We review the lot, setbacks, existing structures, access, utilities, drainage, fire requirements, and likely permit path.
Design and Permit Coordination
Our team coordinates architecture, engineering, energy documents, utility requirements, submittals, comments, revisions, and approved construction documents.
ADU Construction
We manage site work, foundation, framing, building envelope, mechanical, electrical, plumbing, finishes, inspections, and documented changes.
Final Inspection and Closeout
We coordinate corrections and final inspections, complete the punch list, and organize closeout and applicable warranty information.
Google rating 4.6 ★ from 47 public reviews
ADU Project Reviews
Verified public reviews from homeowners who hired Hamilton Exteriors.
Y
Yaku Escobar
via Google
We worked with Hamilton Exteriors to build a small ADU in our backyard. from the beginning the process felt organized and clear. Alex Lee was very responsive and always kept us in the loop, especially during planning and inspections. The crew was consistent and respectful of the space which we appreciated since we were living there during the build. overall everything moved pretty smoothly and the final result looks exactly like what we had in mind. really happy with how it turned out.
R
Rafa Simons
via Google
My family hired Hamilton after seeing the neigbours new ADU, the whole team were super organized and tecnological and also very patient to undestand my needs before starting the construction. But what i like the best is that they are really preocupated with the final result and they kept us well inform with details and everything. Definitely recomend them.
H
H.T.
via Google
I had a great experience working with Hamilton exteriors. From start to finish, their team was professional, responsive, and detail-oriented. They made the entire process smooth and stress-free, keeping me updated every step of the way. The quality of their work really stands out. Everything was done efficiently and exceeded my expectations. If you’re looking for a reliable company that takes pride in what they do, I highly recommend them!
J
Jessica W.
via BBB
The team was professional, communicative, and completed the job in a timely manner. They kept me informed throughout the entire process and arrived as scheduled. The work was well supervised, and the crew paid close attention to every detail. They left the property clean and organized when the job was finished. Overall, the entire experience was smooth, stress-free, and exceeded my expectations. I would definitely recommend their services to others.
Google rating: 4.6/5 from 47 public reviews
Read reviews on Google Mid Market
ADU Costs in Castro Valley
$150,000–$400,000
Median home value in Castro Valley: $1,050,000
Garage conversions: $150,000–$200,000. Detached ADUs: $200,000–$400,000. Junior ADUs: $150,000 and up. Costs depend on size, site conditions, and finishes. Free consultations with detailed project estimates provided.
Get Your EstimateCompare Broad ADU Project Paths
Swipe to compare all options.
| Feature | Detached ADU | Garage Conversion | Junior ADU |
|---|---|---|---|
| Location | Separate backyard structure | Within an existing garage | Within the primary home |
| Existing shell | New foundation and structure | Existing shell requires verification | Existing conditioned structure |
| Main feasibility focus | Lot, access, setbacks, and utilities | Structure, slab, height, and utilities | Interior layout and required shared systems |
| Privacy planning | Separate building and entrance | Entrance and yard relationship | Connection to the primary home |
| Dedicated guide | Detached ADUs | Garage conversions | Start with an ADU consultation |
ADU Financing
Plan Your ADU Project With Approved Credit
Financing may be available for eligible ADU construction projects with approved credit. During feasibility and scope planning, we explain current lender programs, terms, amounts, and payment timing that may apply to the design and construction path.
Ask which current options fit your ADU project scope.
Call Now (650) 977-3351 Frequently Asked Questions
Didn't find the answer to your question? Call Now (650) 977-3351
What areas do you serve?
We serve Northern California across three regions: the San Francisco Bay Area, Greater Sacramento, and the Central Valley. Our published guides cover 14 counties and 88 cities. Our Castro Valley and Sacramento offices support local estimating, permit coordination, and project planning across these regions.
How does billing and financing work?
Financing may be available for eligible projects with approved credit. Available terms depend on lender approval, project scope, and current programs. Your project manager can review the options that apply before work begins. We also accept checks, ACH, and major credit cards.
Are you licensed and insured?
Yes. Hamilton Exteriors holds CSLB License #1078806, active general liability coverage, and the required contractor bond. We also hold GAF Master Elite and James Hardie Elite Preferred credentials. We can provide current proof of coverage before work begins.
Do I need a permit for my exterior project?
Permit requirements depend on the project scope and local jurisdiction. Hamilton Exteriors coordinates required applications, plan documents, corrections, and inspections. Permit fees and allowances appear in the proposal before construction begins.
Why Choose an Architect-Led ADU Builder in Castro Valley?
Most ADU contractors in Castro Valley are tradesmen-only. They build what you tell them to build. Hamilton Exteriors is different. Our projects are led by a licensed architect and general contractor — Alexander Hamilton Li — who brings a designer's eye to every project. This means your ADU is not just structurally sound but also well-proportioned, energy-efficient, and integrated with your existing home and lot. An architect-led approach also reduces coordination risk. The same team handles desi
How much does an ADU cost in Castro Valley?
A detached ADU in Castro Valley typically costs $200,000 to $350,000 for a 600 to 800 square foot unit with mid-range finishes (Hamilton Exteriors 2024-2026 project data). Garage conversions run $150,000 to $200,000. Junior ADUs start around $150,000. Permit fees through Alameda County add $5,000 to $15,000. These prices include design, engineering, permits, and construction.
Do I need a permit for an ADU in Castro Valley?
Yes. All ADUs in Castro Valley require a building permit through the Alameda County Building Department. California state law requires approval or denial within 60 days. We handle the entire permit process.
How long does it take to build an ADU in Castro Valley?
Plan on 6 to 12 months total from design start to move-in. Design and permitting take 2 to 4 months. Construction runs 8 to 14 weeks for detached ADUs and 10 to 14 weeks for garage conversions. The permitting phase is the most variable, though state law requires a 60-day decision.
Can I build an ADU on my Castro Valley property?
If you own a single-family home in Castro Valley, you almost certainly can. California state law (AB 68, SB 13) overrides most local zoning restrictions. Your lot must meet setback requirements — typically 4 feet from side and rear property lines. We evaluate your lot during a free on-site consultation.
Your Next Step
Plan Your Project With a Clear Scope
Financing may be available with approved credit for eligible projects.
Tell us about your project. We usually call within 3–5 minutes to schedule the next step.
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21634 Redwood Rd Unit F, Castro Valley, CA 94546
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